Take everything with a grain of salt. Invest in a cheap park "chain checker". When people say they have only ridden their bike a couple of times and the chain has stretched out .1...you know there's an issue, LOL.
Depreciation will vary from brand to brand and from model to model within a brand. It's tough to say. I've noticed that Jamis has a lower resale value than something like a Specialized...but I have alot of confidence in some of the models.
Just make sure and check out the bike thoroughly before pulling the trigger. Check the hubs to make sure they're not loose. Fork bushings, headset, bottom bracket, drive train, etc.